I have to disagree with others here. One can always write something, but writing what one knows is a key phrase bandied around by teachers and coaches. If you never have had a serious relationship, you can end up using a lot of clichés and stereotypes in your story, because all you can do is copy from others. Hands-on life experience does help for creating something fresh and authentic sounding. My advice would be to write something else now, and then when you're older and have had significant romantic experience, try your hand at the genre.
Honestly, there is a case to be made for your lone status being MORE conducive to romance.
You are on the outside looking in right now, likely you want in, want in bad, want someone to hold, that person you feel safe cuddling with.
So you use your own desires as the basis for your lovers.
On the flipside, people who have been in relationships for a while often lose touch with romance. Just because you are in a relationship doesn't mean you know a damned thing about romance.
A starving man has a different perspective than the full man.
You are the starving woman. Write what you know.
Sex is not romance. Even a relationship is not romance.